This steady accumulation of the scientific consensus ultimately became too big for politicians to ignore. Congress and the president responded, and in 1990, they enacted amendments to the Clean Air Act instituting a cap-and-trade system to control acid rain pollutants. The system has worked, and acid rain has been dramatically reduced.

Perhaps the most obvious reason for stasis in Washington is when public opinion runs counter to a policy change. Few elected officials will risk the wrath of voters on hot-button issues.

In the gun debate, large majorities of voters favor particular changes to laws, such as banning assault weapons or high-capacity ammunition magazines. But polls also show that they’re skeptical that such changes would affect the right people or reduce gun crime. A Washington Post poll last year found that 57 percent of Americans thought that tougher enforcement of existing laws was a better way to reduce gun crime versus 29 percent who favored passing new laws. And focus groups have long found that many voters are turned off by elected officials who call for new gun laws. So politicians are generally right to think that gun control makes bad politics.

Yet voters reserve the right to change their minds. When he ran for president in 1992, Bill Clinton vowed that he would lift the ban on gay men and women serving openly in the military, and upon taking office, he tried to do it. But he met opposition in the Pentagon and in Congress, fueled by polls showing that the public opposed the change. Clinton was forced into a face-saving compromise — “don’t ask, don’t tell” — that kept the ban in place.

Over time, though, the politics of gay rights began to shift. By 2010, when Obama persuaded Congress to scrap “don’t ask, don’t tell” and lift the ban, a majority of voters said they supported its repeal. And even members of Congress who opposed the move mounted only a halfhearted campaign when the measure came up for a vote.

What this recent history shows is that Washington is rarely shocked into action. Galvanizing events can change the tone briefly: Congress adopted bipartisan seating at the State of the Union address after the shooting of Rep. Gabrielle Giffords (D-Ariz.), for instance. But even really big surprises rarely lead to major political change. We no longer see many Sputnik moments.

Rather, change is slow and cumulative. It takes time for a problem to boil over, for expert consensus to pierce the fog of denial, for public opinion to shift decisively.

We see this process today on the same issues: Social Security, the environment and gay rights. The AARP still opposes significant changes to Social Security, but as the potential for catastrophic fiscal failure becomes more real, prospects for reform are improving. The scientific consensus is solid on global warming, and as the weather grows more destructive, demands for action will likely become too loud to ignore. And even though the public has voted in 31 states to ban same-sex marriage, the national mood is changing, raising the odds that the Defense of Marriage Act eventually will be swept away.

So too on guns. It may be that nothing revolutionary or evolutionary will soon alter the gun debate. There are nearly 300 million guns in private hands in this country, and the Supreme Court has affirmed that the Second Amendment confers the right to own guns on most law-abiding adults. And the power of the NRA is real.

But when we ran a group called Americans for Gun Safety, we found reason for hope. We worked with Sen. John McCain, R-Ariz., following the Columbine tragedy, because he was tired of doing nothing to prevent gun deaths. Together, we wrote legislation that would close a loophole in the law allowing people to obtain guns without background checks at gun shows. It reached the Senate floor in 2004 and passed as an amendment to another bill, but it was scuttled by NRA supporters.

McCain took a lot of grief for his stance. After he appeared in our television ads in Colorado promoting changes to close that state’s gun show loophole, the NRA featured him on the cover of its magazine drawn as the devil.

Sadly, when McCain ran for president again in 2008, he reverted to the NRA’s point of view. He knew it would be virtually impossible for him to win the GOP nomination if he was opposed by the most powerful interest group in conservative politics.

McCain’s heresy on guns was short-lived. But rather than see it as a depressing truth about the power of the NRA, we see it as proof that virtually nothing is impossible in American politics. Even mountains will eventually be moved, but that takes the work of years, not a moment of shock or pain.

Matt Bennett is a co-founder and senior vice president of Third Way. Jonathan Cowan is a co-founder and president of Third Way. Previously, they served as director of communications and president, respectively, of Americans for Gun Safety.